For the 14 crew aboard the Karagöl, a ­Turkish chemical tanker churning through the lawless waters of the Gulf of Aden, it was the moment all seafarers dread: heavily armed Somali pirates were speeding towards the slow-moving cargo vessel, and there was no chance of escape.

The Turkish sailors were swiftly overpowered and the 5,850-ton tanker was diverted to a port in Somalia, where it was held for two months while its owners negotiated a ransom payment.

What the crew could not know was that their ship had been singled out as a target by a network of informers based several thousand miles away – in London. Security officials say well-placed informants in the British capital, the world centre of shipbroking and insurance, gather so much detail on targets that, in the case of the Karagöl, they not only knew its layout, route and cargo, but had spent several days practising the assault.

The attack on the Turkish ship was a sign that the pirates have turned a regional phenomenon into a global criminal business that now reaches into the heart of London's shipping community.

"They made regular calls from the ship to London," said Haldun Dincel, general manager of Turkey's Yardimci shipping company, who negotiated the release of their ship. The calls were made on satellite phones the pirates brought with them.

Speaking by telephone from Istanbul, Dincel said today that London was one of a number of centres the pirates contacted regularly after the tanker had been sailed to the Somali coast and senior gang members had boarded and taken control. "Every day the chief of the pirates got in touch with people from London, Dubai and some from the Yemen," he said.

At least one of the four or five major pirate groups that are now carrying out the attacks has London-based "consultants" to help them choose their targets, according to a European military intelligence report leaked to Spain's Cadena SER radio station yesterday.

The report has been circulated around those countries, including Britain, that are involved in the European Union's Operation Atalanta to protect ships against piracy in the area. It indicated that the hijacking of at least three vessels, including the Karagöl, the Greek cargo ship Titan and Spanish tuna trawler Felipe Ruano, followed tipoffs from the London-centred network of informers, according to Cadena SER.

In each case, according to the report, the pirates had full knowledge of the cargo, nationality and course of the vessel.

It is not clear who these ­"consultants" were, but Dincel believes they may work inside the industry. "They knew the ­vessel, they knew the cargo, they knew the loading ports, they knew the destination, they knew everything," he said. "The knew their job."

Andrew Mwangura, who heads the East African's Seafarer's Assistance Programme, a piracy monitoring group based in Mombassa, Kenya, said negotiations over hijacked ships often involve Somalis in London. "Not only for the Karagöl, but for many other ships, the negotiations involve people in London," he said.

The EU report said information being passed to the pirates was often extracted from the international organisations that control or track the world's shipping.

The national flag of the vessel was also taken into account when choosing a target, with British vessels apparently being increasingly avoided, the report said.

"We have heard this a lot. It strikes me as plausible," said Graeme Gibbon-Brooks, of Dryad Maritime Intelligence, last night. "They are getting more sophisticated because they are funded by criminal gangs from outside of Somalia."

He warned, however, that while pirates might receive information on individual targets from London and elsewhere it was still difficult to locate a ship in mid-ocean. Pirates were more likely to receive lists of potential targets so they could identify one if they came across it, he said.

Dincel said he suspected the pirates' informers had also infiltrated the authorities who run the Suez canal, enabling them to track the Karagöl's movements from the moment it left the canal.

Dincel himself spoke several times a day to one of two pirate negotiators who had both lived in the US. "One said he had lived there for 10 years," he said. "The other had graduated from a US college. The ship's master also said they were educated people."

Dincel said the chief negotiator had told him over the telephone that all young Somalis wanted to become pirates. "He said that he had a car, money and a house. He has everything and the young people see him, and naturally they ask to be pirates." In January, Yardimci eventually airlifted money to the pirates to secure the release of the Karagöl and its cargo.